1. Field of the Invention
The present invention relates to power tools and, in particular, to power tools of the impact or impulse type.
2. Description of the Prior Art
Power-driven impulse tools, such as impact wrenches, are commonly provided with a clutch mechanism which will operate to deliver torque from a motor-driven input shaft to an output shaft coupled to an associated load, such as a fastener. At low torque, such as when a nut is initially being run down on a stud, the torque is delivered continuously at high speed, but at higher torque levels the torque is delivered intermittently in a series of impulses or impacts.
More specifically, the clutch mechanism typically includes a hammer connected to the drive shaft and an anvil connected to the output shaft, with inter-engaging parts, such as fingers. The hammer is spring-biased axially into engagement with the anvil. When the torque, or resistance of the driven member, reaches a predetermined level the output shaft slows down but the input shaft continues to rotate substantially at the motor speed. The hammer is coupled to the drive shaft by a suitable screw-like camming mechanism, so that the speed differential of the drive and output shafts causes the hammer to retract axially against the bias spring until the clutch fingers on the hammer and anvil disengage. When the clutch is disengaged, the hammer will rotate freely with the drive shaft past the previously-engaged fingers on the anvil and the bias spring will simultaneously urge the hammer axially back into engagement with the next finger on the anvil to create a torquing impact of the hammer against the anvil, after which the hammer will again retract as torque builds up. Thus, torque is delivered in a repeated hammering action.
These prior types of impact tools have a number of disadvantages. First of all, the repeated hammering action is very noisy and generates considerable reaction forces resulting in severe vibration of the tool. The device affords relatively inconsistent torquing, the torque varying with fastener joint mass and stiffness. Also, the mechanism does not afford easy and precise control of the limiting torque.